The present invention relates to silicon pressure sensors, and in particular to sensors formed with silicon bonded to glass.
For precision applications, pressure sensors fabricated using silicon micromachining are normally anodically bonded to a glass substrate. This substrate provides stress isolation between the silicon sensing element and the housing when the wafer is diced up into individual sensing elements.
Silicon pressure sensors are known to be more sensitive to backside loading of pressure than top-side loading. In part this is due to the effects of the sidewalls acting as additional surfaces to exert pressure against. Topside loading tends to compress these surfaces while backside loading tends to stretch these surfaces, making it easier for the part to blow out.
Analysis has traditionally shown that by rounding the interface between the diaphragm area and the sidewalls, a good portion of the problems of backside loading can be addressed. Recently, however, as die size has shrunk, a secondary failure has been observed in a number of devices. This failure is that, when the part which is anodically bonded to the glass is overstressed with pressure from the backside, the entire edge of the die blows out This fractures the glass and takes out the entire edge of the die from one edge of the cavity to the other edge of the cavity (see FIG. 2B).
The objective of this invention is to substantially reduce the chance of a blow-out in the corner of the cavity in this anodically bonded structure.